The folks at App.net were kind enough to give me a link good for 200400700 free accounts. If you want to join this awesome service, you can use the link above to sign up for free (while supplies last).
Update: The free invite codes are now used up.
And once you start looking for ADN clients, I use just one: Riposte on the iPhone. I hear Felix is also fantastic. And on the Mac, Kiwi is all the rage with the cool kids.